While face masks reduce the spread of Coronavirus, their filtering capability is adversely affected by repeated coughing, according to a study which recommends PPE for healthcare workers including helmets with built in air filters, and face shields.
Scientists including Dimitris Drikakis and Talib Dbouk from the University of Nicosia in Cyprus, used computer models to map out the expected flow patterns of small droplets released when a face mask wearing person cough repeatedly.
Earlier computer simulations the researchers showed that droplets of saliva can travel 18 feet in five seconds when an unmasked person coughs.
In the study published in the journal Physics of Fluids, they used an extended model to consider the effect of multiple cycles of coughing on the filtering efficiency of face masks.
According to the study, while face masks reduce the spread of airborne droplets, their effectiveness is adversely affected by repeated coughing, as might happen when an individual is ill.
Scientists added that the use of face masks does not provide complete protection, and social distancing remains important during COVID-19 pandemic.
In the study, the researchers modelled a sequence of coughs by applying several cycles of forward directed velocity pulses to the initial droplets.
Researchers then performed numerical simulations to account for droplet interactions with the porous filter in a surgical mask. According to the scientists, even when a facemask is worn, some droplets can travel a considerable distance, up to one metre, during bouts of mild coughing.
Without the facemask, they said water droplets travel twice as far, adding that wearing a face mask will help. While face masks also decrease the number of water droplets leaking out the sides of the mouth, they fail to eliminate this entirely, the researchers said.
The water droplets sizes fluctuate and change continuously during cough cycles as a result of several interactions with the face and mask, said Drikakis.
Facemasks decrease the water droplet accumulation during repeated cough cycles. However, it is still unclear whether small droplets or large droplets are more infectious, Dbouk explained.
Based on the conclusions, researchers recommended much more complete PPE for healthcare workers, including helmets with built in air filters, disposable gowns, double sets of gloves and face shields.
They also urged regulatory authorities and manufacturers to consider new criteria for assessing mask performance which account for flow physics and cough dynamics.